核磁共振骨皮质成像关键技术研究进展

Review of the ultrashort echo time magnetic resonance imaging of cortical bone

骨质量尤其是骨皮质质量的评价方法对骨病的诊断和治疗有重要意义.随着社会快速老龄化,如何非侵入地获得准确实用的骨质量评价指标已成为医学物理领域亟待解决的热点问题.目前有多种骨质量评价方法,其中双能X射线吸收法获得的骨矿密度值是评价骨质量的现行金标准,但这个参数有明显缺陷,如不能反映骨皮质中的有机基质、微结构、孔隙度及灌注等情况,所以不能准确诊断骨质疏松和预测骨折等疾病.由于骨的磁共振信号衰减极快,所以常规磁共振成像技术不能探测到骨的信号.近年来随着理论、方法和设备的不断进步,超短回波磁共振骨成像成为可能.本文简要介绍超短回波磁共振骨成像的基础物理理论,结合作者所在实验室的研究工作对各类定性及定量超短回波磁共振骨皮质成像新方法进行综述,总结各类方法的特点、适用范围及不足,指出进一步研究的方向、重点及步骤,对超短回波磁共振成像在骨质量评估方面的理论研究及工程应用具有指导意义.

The evaluation of bone quality, especially cortical bone, is very important for diagnosing and treating the bone diseases. Because of the rapidly aging population of the global society, noninvasively, precisely and feasibly evaluating the bone quality has become a hot topic in the contemporary medical physics studies. Among the several available methods of evaluation, the bone mineral density （BMD） measured with dual-energy X-ray absorptiometry is currently considered to be the gold standard in clinical applications. However, the BMD is limited by its incapability of assessing the organic matrix, microstructure, porosity and perfusion of bone. In addition, the BMD can neither provide a definite diagnose of osteoporosis nor predict fractures precisely. Cortical bone shows near zero signal with all conventional clinical magnetic resonance imaging （MRI） sequences, because of the rapid decay of the magnetic resonance signal in the bone. Due to the recently developed theories, methods and hardware, ultrashort time echo （UTE） sequences with nominal TE of less than 100 μs have aroused the increasing research interest. In this paper an introduction to the basic physics of UTE MRI of cortical bone is presented. The newly proposed qualitative and quantitative UTE MRI methods are reviewed with an introduction to the research work in the authors＇ laboratory. The features, application scopes and limitations of those methods are also summarized. Finally, the authors point out the directions and steps of further studies. The paper will be helpful for understanding theoretical research and the clinical applications of UTE imaging of cortical bone.